This invention relates to lasers and more particularly to an improved narrow bandwidth laser.
There is a need for a tunable laser having a very narrow spectral output bandwidth in applications such as narrow band communication systems and laser photochemistry. In the past, the output wavelength of such a tunable laser has been narrowed, controlled relative to an external reference, and/or stabilized by use of intracavity elements which introduce differential loss, i.e., low level loss in the spectral region at which output is desired and high loss in such region at which output is to be suppressed. For example, the fine tuning of the emission wavelength of a dye laser, such as a Rhodamine 6G dye laser pumped by a frequency-doubled Nd:YAG laser, is achievable by use of a wavelength selective resonator which is in one of the following classes:
(1) resonators including devices for spatial wavelength separation,
(2) resonators incuding devices for interferometric wavelength descrimination,
(3) resonators including devices with rotational dispersion,
(4) resonators with wavelength-selective distributed feedback.
Some specific wavelength selective techniques that have been used in the past in a tunable dye laser and typical resultant linewidths are as follows:
No narrowing--20A
Single prism--5A
Multiple prism--1A
Diffraction grating only--1A
Telescope plus diffraction grating--0.1A
Telescope plus diffraction grating plus Fabry-Perot etalon--0.01A
Birefringent filter--0.1A (multi-element)
Birefringent filter plus Fabry-Perot etalon--0.01A
In general the foregoing wavelength selection techniques involve wavelength dependent angular and/or positional deviation in a resonator which then operates at a wavelength at or near a particular angular or positional alignment. For example, tilting an intracavity prism causes a change in the wavelength for which the resonator is aligned in angle, causing a change in output wavelength. Other examples include a grating (tilt in angle), etalon (tilt in angle or change in gas pressure), and a birefringent filter (rotate plates about the beam axis or change voltage on E-O tuning elements).
Stabilization of a tunable laser to correspond precisely to a desired wavelength generally involves use of an external control system of considerable complexity. A sensor detects either the absolute wavelength or its departure from the desired reference. A feedback loop uses the sensor output to generate a signal which represents the magnitude and direction of the wavelength error. A servo system then positions one or more of the spectral control elements within the resonator to adjust the output to the desired wavelength.
The major difficulties inherent in use of the foregoing techniques are complexity and bulk of the equipment, lack of reliability, high costs, and difficulty in installation.
This invention is directed to laser apparatus which avoids the foregoing difficulties and disadvantages.